Binders in which the crosslinks are formed by subjecting a hydroxyl terminated polybutadiene and diisocyanate to urethane reaction have been disclosed as the binder for polybutadiene composite propellant, particularly propellant using a small amount of binder. Hydroxyl terminated polybutadiene used for such binder is represented by R-45 made by ARCO Chemical Co. Ltd. in U.S.A. and is a polymer having a functional group number of 2.2-2.5 and a number average molecular weight of 2,000-3,000, which is produced by radical reaction. In view of this functional group number value, it is apparent the structure of the nominally bifunctional polymer contains a relatively large amount of polymer having three or more functional groups, so that the content of the polymer having three or more functional groups in the above described hydroxyl terminated polybutadiene varies and therefore even if the amount of diisocyanate added is adjusted depending upon this variation when producing the binder, the physical properties of the produced binder are uneven and the reliability when using for production of propellant as the binder is adversely affected thereby. Furthermore, the formation of the crosslink upon curing of the binder is relatively rapid due to the above described cause and the reaction rate with diisocyanate is rapid because the above described functional groups are primary, so that the pot life in propellant formation is short. When a plasticizer is used in order to obviate this problem, the specific impulse of the propellant is deteriorated.